1. Field of the Invention
The present invention relates to a DC power supply device for use with an apparatus such as a video tape recorder with a built-in camera or the like which needs a plurality of DC voltages.
2. Description of the Prior Art
DC power supplies for use with video tape recorders with built-in cameras are typically arranged to supply a voltage of 5 V to a video system, a voltage of 9 V to a camera system, a voltage of 1.5 V to a drum system, and a voltage of 2 V to a capstan system. One conventional circuit arrangement of such DC power supplies is designed so as not to suffer from load variations as shown in FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the conventional DC power supply has an input terminal 1 for being supplied with a DC voltage of 10 V, for example, a video system power supply block 2, a camera system power supply block 3, a drum system power supply block 4, a capstan system power supply block 5, a video system output terminal 2c, a camera system output terminal 3c, a drum system output terminal 4c, a capstan system output terminal 5c, and a control circuit 6. The video system power supply block 2 has a video system power supply switching circuit 2a having an input terminal supplied with the DC voltage of 10 V from the input terminal 1. The camera system power supply block 3 has a camera system power supply switching circuit 3a having an input terminal supplied with the DC voltage of 10 V from the input terminal 1. The drum system power supply block 4 has a drum system power supply switching circuit 4a having an input terminal supplied with the DC voltage of 10 V from the input terminal 1. The capstan system power supply block 5 has a capstan system power supply switching circuit 5a having an input terminal supplied with the DC voltage of 10 V from the input terminal 1. The DC voltage of 10 V from the input terminal 1 is also applied to the control circuit 6 which comprises an integrated circuit.
The switching circuits 2a, 3a, 4a, 5a turn on and off the supplied DC voltage with switching signals of a frequency of about 500 kHz or lower from the control circuit 6, and supply the switched voltages to respective smoothing circuits 2b, 3b, 4b, 5b of the respective blocks 2, 3, 4, 5.
The control circuit 6 compares DC voltages of 5 V, 9 V, 1.5 V, 2 V from respective output terminals of the smoothing circuits 2b, 3b, 4b, 5b with respective predetermined reference voltages, e.g., a voltage of 5 V for the video system power supply block 2, a voltage of 9 V for the camera system power supply block 3, a voltage of 1.5 V for the drum system power supply block 4, and a voltage of 2 V for the capstan system power supply block 5. The control circuit 6 produces switching signals having pulse durations depending on the differences between the compared voltages, and supplies the produced switching signals to the respective smoothing circuits 2b, 3b, 4b, 5b.
The DC voltages of 5 V, 9 V, 1.5 V, 2 V from the respective output terminals of the smoothing circuits 2b, 3b, 4b, 5b are supplied respectively to the video system output terminal 2c, the camera system output terminal 3c, the drum system output terminal 4c, the capstan system output terminal 5c.
The circuit arrangement shown in FIG. 1 has been mounted on a double-sided circuit board, and the switching signals have had a frequency of 500 kHz or lower. Therefore, ripples, spike voltages, and undesirable radiations can be suppressed to a certain extent by suitably designed wiring patterns on the double-sided circuit board.
However, if the switching signals supplied from the control circuit 6 to the switching circuits 2a, 3a, 4a, 5a have a higher frequency ranging from 1 MHz to 1.2 MHz, for example, in an effort to reduce the size of the DC power supply device, then ripples, spike voltages, and undesirable radiations in the circuit on the double-sided circuit board can be suppressed, but tend to interfere with each other, adversely affecting signal lines and other circuits.
To avoid the above shortcomings, it may be possible to employ large-size capacitors and coils in the DC power supply device. However, use of such large-size capacitors and coils results in an increase in the size of the DC power supply device.